1. Field of the Invention
Embodiments of the present invention generally relate to an improved implementation of clocked standby mode in a digital circuit.
2. Description of the Related Art
Modern integrated circuits (ICs), such as dynamic random access memory (DRAM) devices typically contain one or more memory arrays for storing information. Each bit of information within the array may be stored as a charge within the array. Due to inherent imperfections in storing the charge (e.g., leakage currents), the charges in the array may deteriorate, causing the information stored by the charge to be lost. To avoid this information loss, each cell in the memory arrays must be periodically recharged. The process of recharging the cells in a memory array is referred to as a refresh. The frequency of refresh operations necessary to maintain the data in the memory array is referred to as the refresh rate.
The rate at which charges in a memory array deteriorate may vary with operating characteristics of the IC. For instance, leakage currents which cause charge deterioration may increase with the operating temperature of the IC. In some cases, a high refresh rate may be chosen such that no information is lost from the memory array, regardless of the operating temperature of the IC. However, refreshing memory arrays typically consumes power, and a high refresh rate may lead to high power consumption of the IC device. In some cases, for instance where the IC is used in a battery-operated mobile device, high power consumption may be undesirable.
To decrease power consumption of the IC device and avoid unnecessarily high refresh rates, some ICs may adjust the refresh rate based on the operating temperature of the IC. When the temperature of the IC is high, the refresh rate may be increased. When the temperature of the IC is low, the refresh rate may be decreased. Thus, regardless of the temperature, the refresh rate may remain just high enough to prevent memory loss in the memory arrays. Also, by maintaining a minimum refresh rate necessary for proper functioning of the IC, power consumption of the IC may be minimized. Dynamically adjusting the refresh rate of a memory device may be referred to as temperature compensated self-refresh, or TCSR.
TCSR in an IC may be implemented with a temperature sensor and control circuit. The temperature sensor may periodically measure the temperature of the IC, and the control circuit may adjust the refresh rate of the memory arrays based on the measured temperature. In order to measure the temperature of the IC, the temperature sensor may contain a temperature sensitive diode. The temperature sensor may compare a voltage and/or a current in the temperature sensitive diode to a reference voltage and/or a reference current. Based on the difference between the compared voltages and/or currents, the temperature of the device may be accurately measured.
Each reference voltage and reference current used by a temperature sensor may be generated by a voltage generation circuit. When used to generate a reference voltage and/or reference current, the voltage generation circuit may be referred to as a reference voltage generator. IC devices often operate using various internally generated reference voltages in an effort to reduce sensitivity to fluctuating external voltage supplies. A typical IC, such as a DRAM device, may include many such voltage generation circuits, configured to generate a wide range of reference voltages and output voltages, which may include voltages that are positive with respect to a ground reference (e.g., a boosted wordline voltage or VPP) and voltages that are negative with respect to a ground reference (e.g., a back-bias voltage, VBB, or negative wordline voltage, VNWL).
Each voltage generation circuit on a given device may consume power while generating a reference voltage or an output voltage. In order to conserve the power consumed by the IC device, each voltage generation circuit may be placed in a mode (referred to as a standby mode) where the circuit is selectively enabled and disabled. In the standby mode, the voltage generation circuit may be enabled while the required voltage is being used by the IC device. For instance, if the IC device is a memory device, the voltage generation circuit may be enabled just before the memory device uses the output of the voltage generation circuit to perform an access (e.g., a read, write, or refresh). While the voltage generation circuit is enabled, the voltage generation circuit may consume power and generate the required voltage. When the memory device is not being accessed, the voltage generation circuit may be disabled. While the voltage generation circuit is disabled, the circuit may consume less power and the required voltage may not be generated. Because each access to the memory device may be timed according to a clock (e.g., each access to the memory device may occur on the rising edge of a clock), a clock signal may be used to selectively enable and disable the voltage generation circuit just prior to each access. Accordingly, the standby mode may be referred to as a clocked standby mode (CSM).
In some cases, enabling and disabling the clocked standby mode may cause fluctuations in the reference voltage and or reference current of the reference voltage generator. These fluctuations may occur for short periods after the clocked standby mode is either enabled or disabled and may last until the reference voltage generator has adjusted to the change in the clocked standby mode. Because the reference voltage and/or reference current may fluctuate, the temperature sensor, which uses the voltage and/or current to measure the temperature, may measure an incorrect temperature. If the incorrectly measured temperature is too large, the refresh rate of the IC may be increased, leading to unnecessary power consumption. If the incorrectly measured temperature is too small, the refresh rate of the IC may be unnecessarily decreased, possibly leading to memory loss and/or malfunction of the IC. Thus, temperature sensors which use reference signals affected by a clocked standby mode may measure temperatures incorrectly in some cases, leading to malfunction of the IC.
Accordingly, what is needed are improved methods and apparatuses for measuring the temperature of a device that utilizes a standby mode.